In wireless communication systems, access bursts are commonly used to gain access to system resources. Examples of such bursts are the preambles used for access to the physical random access channel (PRACH) and the physical common packet channel (PCPCH) as proposed for the third generation partnership project (3GPP) wideband code divisional multiple access (W-CDMA) communication system.
To gain access to these channels, users transmit a preamble or signature (preamble) to the base station. The base station broadcasts the available codes and time slots that the preambles can be transmitted. The user increases the power level of the transmitted preamble until the base station detects it or until a maximum transmission power level is reached. Once the base station detects a specific user's preamble an acknowledgement (ACK), or negative acknowledgement (NAK), is sent to the user indicating the availability of the channel.
FIGS. 1A and 1B illustrate two possible user densities and cell sizes that access burst detection is used. FIG. 1A illustrates a small cell 24A with a high density of users, such as in an urban area. The base station 20 services user equipments (UEs) 221 to 2217. To accommodate the large number of users, many preamble codes are used to distinguish between users. FIG. 1B illustrates a large cell 24B with a few users. The base station 20 services UEs 221 to 223. Having few users, only a few preamble codes are required to distinguish between users. However, preamble transmission from users (UE 223) closer to the base station are received with much less delay than from users (222) at the periphery of the cell 24B. Each user synchronizes its transmissions to the received timing of the base station's transmissions. As a result, the roundtrip delay of reception of a user's transmission at the periphery of the cell is much larger than closer users. The base station 20 of FIG. 24B needs to handle these delay spreads. Based on the size of a cell and the user density, access burst detectors at base stations 20 need to differ.
Additionally, other cell parameters may differ. As shown in FIG. 2A, the cell 24 has been divided into six sectors, 261 to 266. The base station 20 also uses transmit and receive diversity in each sector 261 to 266 by using two antenna elements 2811 to 2862. per sector 261 to 266. A preamble transmitted in the cell 24 may be first detected by any one of the antenna elements 2811 to 2862 of any of the sectors 261 to 266. As a result in this arrangement, it is desirable that the base station 20 be capable of detecting any preamble code of the cell by any antenna element 2811 to 2862. By contrast in FIG. 2B, the cell is not sectorized and the base station 20 uses a single omni-direction antenna 28.
One approach to handle these varying conditions is to construct hardware to cover the maximal possible round-trip delay for every possible access code on every supported antenna. However, it is unlikely that this designed for worst possible combination of these parameters would occur. Typically, large cells utilize few access codes and small cells used to cover “hot spot areas” typically require more codes. Sectorization also tends to reduce the number of used access codes. Utilizing a worst scenario hardware design typically results in a significant amount of un-utilized hardware in some implementations or a hardware design that is used to only support implementations close to the worse case.
Accordingly, it is desirable to have a Node-B/base station capable of handling these varying conditions in a flexible manner with efficient utilization of the hardware.